The shadow pvmd
(pvmd') runs on the master host
and is used by the master to start new slave pvmds.
Any of several steps in the startup process (for example,
starting a shell on the remote machine)
can block for seconds or minutes (or hang),
and the master pvmd must be able to
respond to other messages during this time.
It's messy to save all the state involved,
so a completely separate process is used.

The pvmd' has host number 0
and communicates with the master through the
normal pvmd-pvmd interface,
though
it never talks to tasks or other pvmds.
The normal host failure detection mechanism is used to
recover
in the event the pvmd' fails.
The startup operation has a wait context
in the master pvmd.
If the pvmd' breaks,
the master catches a SIGCHLD from it and
calls hostfailentry(),
which cleans up.